This invention relates to the use of optical fibers, and, more particularly, to the structure of a bobbin which supports the optical fiber prior to payout.
Optical fibers consist of strands of optically pure glass fiber processed so that light beams transmitted therethrough are subject to total internal reflection. A large fraction of the incident intensity of light directed into the fiber is received at the other end of the fiber, even though the fiber may be hundreds of meters long. Optical fibers have shown great promise in communications applications, because a high density of information may be carried along the fiber and because the quality of the signal is less subject to external interferences of various types, as compared to electrical signals carried on metallic wires. Moreover, the glass fibers are light in weight and made from a highly plentiful substance, silicon dioxide.
Glass fibers are typically fabricated by preparing a preform of glasses of two different optical indices of refraction, one inside the other, or a single glass composition with a coating that ensures total internal reflection, and then processing the preform to a fiber by drawing, extruding, or other method. The optical fiber is then coated with a polymer layer termed a buffer coating to protect the glass from scratching or other damage. As an example of the dimensions, in a typical configuration the diameter of the glass optical fiber is about 125 micrometers, and the diameter of the optical fiber plus the polymer buffer coating is about 250 micrometers.
For such a very fine optical fiber, the handling of the optical fiber to avoid damage or stresses that might reduce its light transmission properties becomes an important consideration. The optical fiber is typically wound onto a cylindrical or tapered cylindrical substrate, called a "bobbin", having many turns adjacent to each other in a side by side fashion to form a layer. After one layer is complete, another layer of optical fiber is laid on top of the first layer, and so on. The array of wound optical fibers is termed the "fiber pack", and the final assembly of the bobbin and the wound layers of optical fiber is termed a "canister". At a later time when the optical fiber is used, the optical fiber is ordinarily paid out from the canister in an unwinding operation, with the speed of unwinding depending upon the particular application.
It has been found by experience that, where the optical fiber is paid out from the canister in a rapid fashion, the turns of optical fiber must be held in place on the canister with an adhesive that holds the fiber pack together. The adhesive holds each turn of optical fiber in place as adjacent turns and layers are initially wound onto the canister, and also as adjacent turns and layers are paid out. Without the use of an adhesive, payout of the optical fiber may not be uniform and regular, leading to multiple dispenses (payout of two or more layers simultaneously), snags or other irregularities that damage or cause the optical fiber to break as it is paid out from the canister.
There have been observed flaws in some optical fiber packs wound onto bobbins, such as wrinkles on the surface, misaligned and loose turns of optical fibers, and cracks in the adhesive extending from the outer surface of the fiber pack downwardly to the surface of the bobbin. These irregularities become particularly apparent after thermally cycling the canister over a range of temperatures, as is often experienced when the canister is stored for a period of time before use. The irregularities can cause a non-uniform payout of the optical fiber from the canister when in use, leading to fracture of the optical fiber and catastrophic loss of signal transmission.
There has been no explanation or solution for the flaws observed in the optical fiber pack of the canisters, and no approach for avoiding the irregularities. There is therefore a need for an approach to optical fiber storage canisters that avoids irregularities in the fiber pack, ensuring a smooth payout of the optical fiber when used. The present invention fulfills this need, and further provides related advantages.